1. Field of the Invention
This invention relates to a gel package structural enhancement of compression system board connections, and particularly to the mechanical, electrical, and thermo improvements to computer systems that utilize LGA compression connector techniques on system boards.
2. Description of Background
In the present construction of the IBM Z-series systems as well as in other programs such as the IBM P-series, the Multi-chip module (MCM) is compressed to the main system board using a mechanical encapsulated MCM structure and a Land Grid Array (LGA) type connection. There has developed with the advent of large scale computing systems, like these mentioned, that a unique problem can be recognized in the construction of the PCB system board. When the system board is fabricated, a change in thickness is produced in the areas of large arrays of vias or plated thru holes (PTH). With this change in thickness, support for a compression style connection scheme such as a Land Grid Array (LGA) can cause many problems. For one, a significant space or gap will develop between the system board and the supporting fixture or stiffener. This gap would then cause instability or flexing of the board, due to expansion and contraction produced by the thermo properties of the system board. To compound this problem, the arrangement of the high density of PTH in the system board due to the design of the I/O pattern of the MCM would cause an uneven pattern on the system board. In areas of low via density, the board would not contract in size as it would in areas of high via densities. In the present design of IBM's Z and P system series, the array of I/O's on the MCM is divided into four areas. This is done for several reasons, one of which is to permit the fabrication of the LGA connector used between the MCM and the system board. This then causes the system board to have different thicknesses that match the I/O pattern.
The resulting board uses then a unique insulator placed between the board and the holding fixture. At present, laminations of multiple layers of insulator arranged in a stacked pattern are used to compensate for the board's thickness change. This approach does not provide a fully supporting structure and also is limited to what size gap can be filled. If the gap exceeds a predetermined size, then the board would not be acceptable. It would be impossible to have multiple stacked insulator designs for use in the assembly of the computer system. At present the system boards are then subjected to screening so that the gap is of a specific size, so that the presently designed stacked insulator is usable.
Furthermore, to achieve the best mechanical interface and electrical connections, it is required that the system board along with its stiffener be as flat as possible in the LGA area. This requirement can significantly increases the total cost of the delivered boards from the PBC vendor as well as the overall cost to produce the computer system. The stringent requirement for board flatness will result in the sorting of the PBC boards based on measurements made after manufacturing. In addition, to this, to insure a sound mechanical interface, all the supporting mechanical structures such as the MCM mounting hardware, and stiffener have to be controlled to very tight tolerances. At present there is a significant gap between the bottom side of the system board and the stiffener. This is caused by a change in thickness of the system board in the area of the MCM interface.
The main reason for the thickness difference of the printed circuit board (PCB) in the LGA area is due to the number of plated thru holes (PTH) or vias in that area. With a large number of vias in any given area shrinkage occurs on both sides of the PCB board. The larger the number of vias in any area on the board, a corresponding larger shrinkage area is produced. For some MCM designs, the I/O arrays are arranged in separate array groups on the bottom side of the MCM. On the system board, there would be a corresponding arrangement of plated thru vias to permit electrical connections within the system board, and thus the system board will be thicker in the areas where there are fewer or no vias and thinner in areas with a large number of vias. Thus depending on the design for the I/O locations for the MCM it would be possible for a system board to contain a non-uniform thickness or rippled effect in the LGA area. At present, the non-planer area directly under the MCM on the topside of the system board is compensated in part, by the pliable LGA connector structure, but the area under the MCM on the stiffener side is not compensated for at all. Although there is an insulator between the system board and the stiffener, the void created by the thickness change in the LGA area is not compensated for. From mechanical analysis, it has been found that the system board will tend to have a concaved bend or thickness change of up to 0.008 inches and in some instances a large gap of 0.012 inches are created. In the LGA area, the board could also have a profile that is rippled in shape. The shape and size of the bend is variable and not always uniformed, and thus the void cannot be filled without some structural design changes. Studies indicate that the system board tends to form a dish or concave profile in the area of the high density via area.
There have been a number of suggestions to date that include a rigid preformed crowned pad that would be inserted in addition between the insulator and the system board or for additional sheets of Mylar in a stack in the crowned area. Using this preformed crowned pad or any other such design to compensate for the irregularities in the LGA area would work for a dish or concave shape of one size, but not for an uneven or structures with multiple topology differences. With the natural creation of the void or dishing on the system board, great care must be used in the overall design to insure component and mechanical integrity. To help prevent the MCM module from breaking, all the mechanical uncertainties and tolerances have to be controlled in the assembling of the functional package. To insure that the LGA interface structure is in the best electrical contact, i.e. low contact resistance, the support area between the system board and the stiffener must be as rigid and solid as possible. If the mechanical system, for mounting the MCM and the electrical interface, is incorrectly compensated for, the gap on the system board could cause significant module damage or electrical contact problems. This would include cracking of the MCM or poor electrical connection through the LGA interface. At present, all suggestions fall short in the solution of these problems.